The present invention relates to vented fuel elements for nuclear reactors and in particular to devices for delaying the release of fission product gases from the fuel element.
In all nuclear reactors, the fission of fissile fuel produces many radioactive fission products, among which are included in particular the gaseous fission product isotopes xenon 133, xenon 135, xenon 140, krypton 85, and krypton 89. The evolution of such gases results in increased pressure within the fuel element and the consequent swelling of the cladding unless the gases are allowed to escape. Such swelling may be sufficient at times to cause the ultimate strength of the cladding material to be exceeded, with the resulting release of dangerous radioactive material in the form of fuel and short-lived fission products into the coolant of the reactor, as well as exposure of the fissile fuel to the coolant with the possible chemical reaction therewith.
In particular, fuel elements for a fast breeding nuclear reactor are required to have more than 100,000 MWD/T of the degree of burn-up rate in order to attain the low fuel cycle cost of the fast breeding reactor of which fuel elements have expensive costs of fuel material, production, reprocessing, transport, etc. as compared with those of a light-water power reactor. The high degree of burn-up rate will necessarily produce a large quantity of the fission product gases.
In the conventional sealed fuel elements, there have been two countermeasures to an increase in the pressure within the fuel element, one of which is the employment of a large volume and another is the increasing of the thickness of the cladding tube.
The enlargement of the plenum increases the plenum length and, as a result, the whole length of the fuel element becomes larger to increase the reactor mass and the size of reactor construction and reactor housing as compared to elements and reactors without increased plenum size and of the same power rating. The increase in sizes of the reactors, etc. are economically disadvantageous.
On the other hand, an increase in thickness of the tube cladding increases thermal stress thereby reducing the safety of fuel element, in cooperation with the increase in the pressure within the fuel element due to the fission product gases. Further, in this case, the neutron economy is reduced to bring about a bad effect on the breeding ratio.
As mentioned above, for the fuel elements for the fast breeding reactor, which has the high degreee of burn-up rate and high ability, the employment of the sealed fuel element requires severe design conditions in view of economy and technique.
In order to solve the above problems, employment of a vented-to-coolant type fuel element has been investigated in the U.S., Japan, etc. in which fuel element fission product gases are allowed to release into a coolant. There have been invented and proposed different kinds of vent means. For example, there is a vent system wherein a plurality of fuel elements are connected by a single manifold to make a fuel element assembly and a large plenum is provided for each fuel element assembly, whereby the fission product gases are vented to the coolant through the plenum. Another is a system wherein a capillary tube is projected from each fuel element into a cover gas so as to vent the fission product gases to the cover gas.
There are other systems, such as, a mechanical check valve system utilizing a spring, a diving-bell system utilizing a balance between a fission product gas pressure and a coolant pressure, and a porous-plug system utilizing permeability or impermeability of a porous material with respect to the liquid metal i.e. the coolant or sealant. Among these vent systems, the diving bell system and porous plug system are noticed to have the higher possibility of realization than other sytems because they have high reliability and safety owing to their simple construction and no movable parts, and thus further have economy due to no need for complicated attachments.
In the diving bell vent system, there is no problem of material choice and the system has high reliability; however, the vent device per se becomes large and long, so that economy of this system may be remarkably deteriorated.
On the other hand, in the porous plug vent system, it is possible to make the vent system shortest, but there is a severe problem in this system that dry porous material conventionally proposed cannot satisfy the conditions that the porous plug prevent the coolant from flowing into the plenum through the porous plug and also hold up the fission product gases therein as long as possible.
In a usual fast breeding reactor which uses a mixed oxide fuel material, the permeation of a coolant or liquid sodium into the fuel element must be prevented since there occurs a severe problem due to coexistence of the fuel material and the liquid metal. Further, the fission product gases vented from the vent system should be stable nuclides or at least nuclides having a long half life, otherwise radioactivity strength in the cover gas and coolant will increase and, as a result, economy of this system becomes deteriorated since there is necessity of shielding, fission product gas removing apparatus, etc.
In order to develop porous plugs satisfying the above-mentioned conditions, different investigations have been made in many laboratories. As a result, the porous plug material usable for a vent device could not be found out that would satisfy the above requirements. In an attempt, a gas holding-up pressure of 20 psi could be obtained by employing a proous body made of sintered stainless steel having a mean pore size of 1 micron; however, it has also been found that this material is poor in its ability to prevent flow of the coolant into the fuel element. Therefore, when the reactor is stopped which will shrink the gas volume, so that the pressure within the cladding tube becomes lower than that of the coolant surrounding the fuel element, the coolant flows into the cladding tube.